Which among the following is a pair of functional isomers?

What is isomerism? Explain with a suitable example.

Which one of the compounds shown below is not an isomer of the others?

The starting material is propene $(CH_3-CH=CH_2)$. The reaction with $H_3O^+$ (acid-catalyzed hydration) follows Markovnikov's rule to give $A$ (propan$-2-$ol,$CH_3-CH(OH)-CH_3$). The reaction with $(i) BH_3/THF$ followed by $(ii) H_2O_2/OH^-$ (hydroboration-oxidation) follows anti-Markovnikov's rule to give $B$ (propan$-1-$ol,$CH_3-CH_2-CH_2OH$). What is the relationship between products $A$ and $B$?

Cyanides exist in:

What do the following pairs of structures represent?